Electrosurgical generators are used for surgically treating the tissue and bodily fluids of a patient. One of the important features of an electrosurgical generator is the ability to control the output power. Surgeons prefer to work with electrosurgical generators that can deliver a controlled level of power to the tissue. This is because a controlled power level is safer and more effective in surgery.
One of the factors that effects the output power is the electrical load on the generator that is presented by the tissue and bodily fluids of the patient. In particular, the impedance of the tissue that is being treated will change as electrosurgical energy is applied. It is therefore desirable for electrosurgical generators to monitor the impedance of the load and adjust promptly the output power accordingly and effectively. As different types of tissue and bodily fluids are encountered the impedance changes and the response time of the electrosurgical control of output power must be rapid enough to seemlessly permit the surgeon to treat the tissue. Moreover the same tissue type can be desiccated during electrosurgical treatment and thus its impedance will change dramatically in the space of a very brief time. The electrosurgical output power control has to respond to that impedance change as well.
Designers of electrosurgical generators define the behavior of the output power according to power curves. These curves describe the RMS power delivered to the patient as a function of impedance of the load. It is possible to divide the power curve into regions based on the impedance level of the load as measured. At low impedance levels, the electrosurgical generator may be designed to limit the current flowing to the patient. At high impedance levels, the electrosurgical generator may by design be voltage limited. In other ranges of impedance, the electrosurgical generator may be designed to maintain a constant level of RMS power supplied to the patient.
A control apparatus for an electrosurgical generator may be required to change its method of power regulation based on the region of impedance. For example, the generator may change from a current limiting mode, to a constant power mode, and then to a voltage limiting mode. Rapid computational methods are required to affect this kind of mode switching and response to varying tissue impedance during electrosurgery.